Add external calendars feature: display read-only ICS calendars alongside CalDAV calendars

- Database: Add external_calendars table with user relationships and CRUD operations
- Backend: Implement REST API endpoints for external calendar management and ICS fetching
- Frontend: Add external calendar modal, sidebar section with visibility toggles
- Calendar integration: Merge external events with regular events in unified view
- ICS parsing: Support multiple datetime formats, recurring events, and timezone handling
- Authentication: Integrate with existing JWT token system for user-specific calendars
- UI: Visual distinction with 📅 indicator and separate management section

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/migrations/006_create_external_calendars_table.sql

@@ -0,0 +1,16 @@
+-- Create external_calendars table
+CREATE TABLE external_calendars (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id INTEGER NOT NULL,
+    name TEXT NOT NULL,
+    url TEXT NOT NULL,
+    color TEXT NOT NULL DEFAULT '#4285f4',
+    is_visible BOOLEAN NOT NULL DEFAULT 1,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    updated_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    last_fetched DATETIME,
+    FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
+);
+
+-- Create index for performance
+CREATE INDEX idx_external_calendars_user_id ON external_calendars(user_id);